This invention relates to high-frequency electrical test instruments and, more particularly, to test instrumentation operable over a wide range of frequencies of interest. Specifically, the invention is directed to adjustment of manual control settings used in broadband test instruments and provides, in one embodiment, a radio-frequency (RF) and microwave spectrum analyzer whose frequency measurement settings for displaying measured data can be adjusted and in which previously measured data can be recast based on the adjustments and displayed as a precursor or estimate prior to or while a new measurement is being performed, thereby improving user efficiency and enhancing overall measurement throughput.
Signal analysis, simply defined, is the extraction of information from an electromagnetic signal, whether performed in the frequency or time domain. The most common time domain signal analyzer is the oscilloscope. In the frequency domain, the signal analyzer is the network analyzer or spectrum analyzer. These analyzers typically display the raw, unprocessed signal information, that is, voltage, power, period, waveshape, sidebands, and frequency.
By way of example, the spectrum analyzer is widely accepted as a general purpose test instrument capable of performing a broad range of measurements in the frequency domain. Examples of such spectrum analyzers are the HP8568 and HP8566 spectrum analyzers and HP71000A Modular Spectrum Analyzer available from Hewlett-Packard Company, Signal Analysis Division, Rohnert Park, Calif.
One technique to perform frequency domain measurements with a spectrum analyzer is known as the swept-tuned technique. The swept-tuned frequency spectrum analyzer can be either a tuned filter or a heterodyned receiver. Swept spectrum analyzers are used to measure a variety of characteristics of signals. There are many measurements which can be performed with a spectrum analyzer in response to a transmitted or received signal, where measurement of frequency, power, distortion, gain, and noise characterize a transmitter or receiver system.
FIG. 1 shows a generalized superheterodyne swept-tuned spectrum analyzer. An incoming signal mixes with a local oscillator (LO) signal, and when a mixing product equals the intermediate frequency (IF), this signal passes through to a peak detector. The peak detector output is amplified to cause a vertical deflection on a CRT display. The synchronism between the horizontal frequency axis of the CRT display and the turning of the local oscillator is provided by a sweep generator which both drives the horizontal CRT deflection and tunes the LO.
Test instruments with graphic display, such as oscilloscopes, network analyzers, and spectrum analyzers, typically have user controls to adjust the parameters of the measurement being taken. The graphic display provides the result of the last measurement taken. When the user adjusts a control setting by means of a knob or button, a new measurement is taken and the graphic display is updated to reflect the new measured data.
In the case of a swept spectrum analyzer, a consideration in narrowing resolution bandwidths for better resolution is the time required to sweep through the filters. Since these filters are bandwidth limited, they require a finite time to respond fully. How much time they are given depends upon how far and how fast the spectrum analyzer is tuned, that is, the effective rate of the analyzer is a function of frequency span and sweep time. For a given frequency span, the sweep time is inversely proportional to the square of the IF bandwidth (or the product of the IF and video bandwidths). Many spectrum analyzers have the capability of setting the sweep time automatically based on the span and bandwidth settings.
The sweep rate is typically variable, and can require considerable time, for example, a minute, to perform a swept measurement. However, changes in control settings are restricted to periods between measurements. For example, in the HP71000A Modular Spectrum Analyzer, requesting a change of the center frequency of the displayed spectrum has previously resulted in a delay for the analyzer to retune and perform another data sweep before displayed data is affected by the adjustment of center frequency.
Considered in more detail, FIG. 2 illustrates the traditional method of how present signal analyzers respond to adjustments to measured data. For example, when a control setting of a swept spectrum analyzer is changed, the feedback on the graphic display is delayed until the signal is swept again. Currently available spectrum analyzers either do not address the problem, or they restart the sweep to reduce the delay. It would be highly desirable to enable adjustments in control settings prior to or during a subsequent measurement (sweep) after data from an initial measurement is observed by the user.